Camptothecin, a plant alkaloid derived from the Chinese Camptotheca acuminata tree, was added to the National Cancer Institute's natural products screening set in 1966. It showed not only strong antineoplastic activity, but also poor bioavailability and toxic side effects. After thirty years of modifying the camptothecin scaffold, two camptothecin derivatives emerged and are now approved for clinical use. The first derivative is topotecan (also called Hycamptin®; GlaxoSmithKline; London, England), which can be used to treat solid brain, lung and ovarian tumors. The second derivative is irinotecan (also called CPT-11 and Camptosar®; Pfizer; New York City, N.Y.), which can be used to treat solid brain, colon and lung tumors, as well as refractory forms of leukemia and lymphoma.
The sole target of camptothecin and camptothecin-derived antineoplastic agents is human topoisomerase I. Camptothecin and camptothecin-derived antineoplastic agents bind to covalent topoisomerase I-DNA complexes and prevent re-ligation of broken single DNA strands, effectively trapping it on the DNA. Such immobilized macromolecular adducts act as roadblocks to the progression of DNA replication and transcription complexes, causing double-strand DNA breaks and apoptosis. Structural studies have established that camptothecin and other camptothecin-derived antineoplastic agents stack into duplex DNA, replacing the base pair adjacent to the covalent phosphotyrosine linkage. See, Chrencik et al. (2004) J. Mol. Biol. 339:773-784; and Staker et al. (2002) Proc. Natl. Acad. Sci. USA 99:15387-15392. Re-ligation of the nicked DNA strand is prevented by increasing the distance between the 5′-hydroxyl and the 3′-phosphotyrosine linkage to >11 Å. Because neoplastic cells grow rapidly, camptothecin and other camptothecin-derived antineoplastic agents impact these cells more significantly than normal cells and tissues.
Camptothecin-derived antineoplastic agent efficacy, including that of camptothecin, is limited by a delayed diarrhea that follows its administration by about two to four days. For example, “reactivation” of SN-38G, a glucuronidated inactive metabolite of irinotecan, to SN-38, its active metabolite, by β-glucuronidases of enteric bacteria kills intestinal epithelial cells and causes a dose-limiting diarrhea. See, e.g., Matsui et al. (2003) Surg. Oncol. Clin. N. Am. 12:795-811; and Tobin et al. (2003) Oncol. Rep. 10:1977-1979.
While broad-spectrum antibiotics have been used to eliminate enteric bacteria from the gastrointestinal tract prior to irinotecan treatment to reduce reactivation, this approach has several drawbacks. First, enteric bacteria (i.e., normal flora) play essential roles in carbohydrate metabolism, vitamin production and the processing of bile acids, sterols and xenobiotics. Thus, a partial or complete removal of enteric bacteria is not ideal for subjects already challenged by neoplastic growths and chemotherapy. Second, the elimination of the symbiotic enteric bacteria from even healthy subjects significantly increases risk of infection by pathogenic bacteria, including enterohemorrhagic Escherichia coli and Clostridium difficile. Third, bacterial antibiotic resistance is a human health crisis, and the unnecessary use of antibiotics is a significant contributor to this crisis.
Likewise, weak/non-selective β-glucuronidase inhibitors such as saccharic acid 1,4-lactone can be administered to reduce reactivation. These inhibitors, however, are only partially effective in preventing reactivation of glucuronidated metabolites of camptothecin and other camptothecin-derived antineoplastic agents. Fittkau et al. (2004) J. Cancer Res. Clin. Oncol. 130:388-394. Additional non-specific inhibitors of β-glucuronidase include certain divalent cations (e.g., Cu2+ and Zn2+), galacturonic acid and glucuronic acid. Naleway, “Histochemical, spectrophotometric, and fluorometric GUS substrates” 61-76 In: GUS Protocols: Using the GUS Gene as a Reporter of Gene Expression (Gallagher ed., Academic Press 1992); and Handbook of Enzyme Inhibitors, Part A (Zollner ed., 2nd ed. 1993).
For the foregoing reasons, there is a need for alternative compositions and methods for inhibiting bacterial β-glucuronidases and for attenuating reactivation of glucuronidated metabolites of camptothecin and other camptothecin-derived antineoplastic agents or any other glucuronidase-substrate agents or compounds.